The locking of intramedullary nails is known in the art, e.g., locking screws are introduced into the transverse boreholes of the intramedullary nail either with the help of an imaging method (X-ray control) or a fairly complicated targeting device. In both cases, a certain targeting inaccuracy is unavoidable, that is, the tip of the screw cannot be aligned exactly coaxially with the middle axis of the transverse borehole and, instead, deviates therefrom by a certain amount. So that the locking screw ends up in and can be passed through the transverse borehole in spite of this targeting error, the external diameter of the screw is underdimensioned relative to the diameter of the transverse borehole. If the targeting accuracy remains in the range of this underdimensioning, the locking screw can be passed through the transverse boreholes without problems in spite of the targeting error. However, because of the underdimensioning, there is a certain clearance between the locking screw and the transverse borehole.
This clearance defines the amount by which the main fragments of the bone, which are fixed in the corresponding locking hole by means of locking screws, can move relative to the nail and, accordingly, because of the rigidity of the nail, also relative to other main bone fragments fastened with the same nail. Together with the flexibility of the material and of the overall device, this may cumulatively assume a magnitude, which prevents successful healing or delays it significantly. Admittedly, this clearance is unavoidable in order to guarantee the applicability of the locking for the surgeon. However, it is clinically undesirable for certain indications, such as metaphysical fragments.
Even nails with a full cross section, which may have an internal thread in the locking hole, are not without clearance. The internal thread merely prevents the nail from moving axially on the locking screw.